fix: Fix deadline header with correct instant value (#551)

[sebsto]

Instant's value now correctly prints as an EPOCH number

Motivation:

A regression was introduced by #540. The HTTP headers returned by LocalServer contained an invalid representation of the Lamba Deadline.

See #551

Modifications:

• Add CustomStringConvertible to LambdaClock.Instant to just print the Int64 value
• add a unit test

Result:

The runtime works correctly with the new LambdaClock

[sebsto]

As all CI check pass, and this is a trivial bug fix, I will merge without further review.

[Copilot]

Pull Request Overview

This pull request fixes a regression in the Lambda deadline header representation by implementing CustomStringConvertible for LambdaClock.Instant to correctly output epoch values as plain numbers instead of the default struct representation.

• Adds CustomStringConvertible conformance to LambdaClock.Instant with a description property that returns the epoch milliseconds as a string
• Includes a comprehensive unit test to verify the string representation contains only numeric characters and can be converted back to a valid instant

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Sources/AWSLambdaRuntime/LambdaClock.swift	Implements CustomStringConvertible with description property returning epoch milliseconds
Tests/AWSLambdaRuntimeTests/LambdaClockTests.swift	Adds unit test validating instant string representation as numeric epoch value

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